阅读理解

    It is Sunday today. It is my grandma's birthday. Father, mother and I go to visit her.

    She lives with my uncle. In the morning we buy a birthday cake and some fruits and go to her house at ten o'clock.

    We spend about half an hour getting there. My grandma and uncle are very glad to see us. We give the presents to grandma and say“Happ y birthday to you! ”She is very happy. Then we sit down to talk. My uncle can cook.

    The meal is very nice. We all like it very much. After lunch, I play with my uncle. At four o'clock, we say goodbye and go home.

下列哪一历史现象与北魏孝文帝改革互为因果关系（    ）